1. Field of the Invention
The present invention relates a solid state image pickup device referred to as a linear sensor (line sensor) or an area sensor and more particular to a solid state image pickup device suitable to use as an image sensor for a bar-code reader for reading a bar code appended to a medium such as a commercial product.
2. Description of Related Art
FIG. 10 shows an example of a solid state image pickup device, for example, a CCD linear sensor. The CCD linear sensor comprises a sensor row 102 having a plurality of light-receiving regions 101 arranged in one row for converting an incident light into signal charges of an amount corresponding to the amount of the light and storing them and a charge transfer register 104 comprising a CCD for transferring in one direction the signal charges read out by way of a readout gate 103 from each of the light-receiving regions 101 of the sensor row 102.
The readout gate 103 reads out the signal charges stored in each of the light-receiving regions 101 of the sensor row 102 simultaneously upon application of a readout pulse .phi.ROG. The charge transfer register 104 transfers the signal charges under 2-phase driving by 2-phase transfer clocks .phi.H1 and .phi.H2.
A charge/voltage conversion section 105, for example, of a floating diffusion constitution is formed at the final stage of the charge transfer register 104 for detecting the transferred signal charges into a voltage. The output voltage of the charge/voltage conversion section 105 is led out by way of a buffer 106 from an output terminal 107 as a CCD output.
The CCD linear sensor having the above-mentioned constitution is used as an image sensor of a bar-code reader for reading a bar code appended, for example, on a medium such as a commercial product.
By the way, since the linear sensor is in a thermally balanced state before turning on of a power supply and, the light-receiving regions 101 are overflown with charges in the balanced state, pixel signals cannot be read out normally owing to the presence of unnecessary initial charges immediately after the turning on of the power supply (rising of power supply). That is, the pixel signals cannot be read out normally unless the unnecessary charges are discharged, so that start up of the device is delayed.
Since such unnecessary initial charges are increased as the aperture of the sensor section (sensor area) is greater, unnecessary charges in the light-receiving regions 101 cannot be discharged during usual transfer operation unless charges are read-out and transferred repeatedly for several to several tens of lines. Particularly, in a case of a linear sensor applied to a bar-code reader, since the aperture size of 1 pixel is as large as 14 .mu.m.times.200 .mu.m and, therefore, the initial charges in the light-receiving regions 101 also accumulated in a great amount upon rising the power supply, so that the start up of the device till it operates normally after discharging the initial charges is extremely delayed. By the way, the aperture size for one pixel of a usual linear sensor is smaller, for example, as about 7 .mu.m.times.7 .mu.m or 14 .mu.m.times.14 .mu.m as compared with a linear sensor used for the bar-code reader.
However, in a case of the bar-code reader, since the power supply is kept off during a stand-by state and reading has to be started instantly when a user turns on a power supply switch immediately before the use, so that it is necessary to rapidly read a bar code when the power supply turns on.
In view of the above in the existent CCD linear sensor, as shown in FIG. 11, it is adapted to rapidly discharge the initial charges by high speed transfer of the signal charges by making the frequency for the .phi.H1 and .phi.H2 higher than the frequency in usual transfer for a certain period of time after turning on of the power supply.
However, in the linear sensor of the structure in which the initial charges are discharged in a high speed transfer mode upon turning on of the power supply as described above, since two kinds of transfer clocks .phi.H1 and .phi.H2, that is, at high frequency for high speed transfer mode and at low frequency for usual transfer mode are necessary, so that a timing generator for generating such clocks is necessary to bring about a problem of complicating the constitution and increasing the cost.
Further, since there is also a limit for increasing the frequency for the transfer clocks .phi.H1 and .phi.H2 in the high speed transfer mode, there is a limit for shortening the period of time to transfer to the normal operation after turning on of the power supply.